This invention relates generally to a hybrid human/porcine factor VIII and methods of preparation and use thereof.
Blood clotting begins when platelets adhere to the cut wall of an injured blood vessel at a lesion site. Subsequently, in a cascade of enzymatically regulated reactions, soluble fibrinogen molecules are converted by the enzyme thrombin to insoluble strands of fibrin that hold the platelets together in a thrombus. At each step in the cascade, a protein precursor is converted to a protease that cleaves the next protein precursor in the series. Cofactors are required at most of the steps. In its active form, the protein factor VIII is a cofactor that is required for the activation of factor X by the protease, activated factor IX.
Factor VIII or antihemophilic factor was noticed in plasma and named in the 1930s. In the 1940s, a deficiency in factor VIII was associated with the clotting disorder hemophilia A. Factor VIII was found to be X-linked and was hypothesized to be a protein. Work involving bovine, human, and porcine plasma identified factor VIII as a protein in the 1980s, though its definitive cellular source remains uncertain.
Precisely how factor VIII functions in blood coagulation is unknown. It is known that factor VIII is activated to factor VIIIa proteolytically by thrombin or factor Xa. In combination with calcium and phospholipid, factor VIIIa makes factor IXa a more efficient activator of factor X by an unknown mechanism.
People deficient in factor VIII or having antibodies against factor VIII who are not treated with factor VIII suffer uncontrolled internal bleeding that may cause a range of serious symptoms, from inflammatory reactions in joints to early death. Severe hemophiliacs, who number about 10,000 in the United States, can be treated with infusion of factor VIII, which will restore the blood's normal clotting ability if administered with sufficient frequency and concentration. The classic definition of factor VIII, in fact, is that substance present in normal blood plasma that corrects the clotting defect in plasma derived from individuals with hemophilia A.
Several preparations of human plasma-derived factor VIII of varying degrees of purity are available commercially for the treatment of hemophilia A. These include a partially-purified factor VIII derived from the pooled blood of many donors that is heat- and detergent-treated for viruses but contains a significant level of antigenic proteins; a monoclonal antibody-purified factor VIII that has lower levels of antigenic impurities and viral contamination; and recombinant human factor VIII, clinical trials for which are underway. Additionally, a preparation of partially-purified porcine factor VIII is available to treat patients with inhibitors to human factor VIII, i.e., those who have circulating antibody molecules that bind and neutralize human factor VIII.
Hemophiliacs require daily replacement of factor VIII to prevent the deforming hemophilic arthropathy that occurs after many years of recurrent hemorrhages into the joints. However, supplies of factor VIII concentrates have never been plentiful enough for treating hemophiliacs adequately because of problems in commercial production and therapeutic use. For example, the commonly used plasma-derived is difficult to isolate and purify, is immunogenic, and requires treatment to remove the risk of infectivity from AIDS and hepatitis viruses. Recombinant human factor VIII may lessen the latter two problems. Porcine factor VIII may also present an alternative, since human factor VIII is unstable at physiologic concentrations and pH, is present in blood at an extremely low concentration (0.2 .mu.g/ml plasma), and its specific clotting activity is low, compared with porcine factor VIII.
Since many inhibitors of human factor VIII react less strongly with porcine factor VIII, porcine factor VIII is currently used to correct factor VIII deficiency in patients under conditions in which they do not respond to infusions of human factor VIII. A limitation of porcine factor VIII is the development of inhibitory antibodies to it after one or more infusions.
The problems associated with the commonly used, commercially available, plasma-derived factor VIII have stimulated significant interest in the development of a better factor VIII product. There is a need for a more potent factor VIII molecule so that more units of clotting activity can be delivered per molecule; a factor VIII molecule that is stable at a selected pH and physiologic concentration; a factor VIII molecule that is less apt to produce inhibitory antibodies; and a factor VIII molecule that evades immune detection in patients who have already acquired antibodies to human factor VIII.
It is therefore an object of the present invention to provide a factor VIII that corrects hemophilia in a patient deficient in factor VIII or having inhibitors of human factor VIII.
It is a further object of the present invention to provide methods for treatment of hemophiliacs.
It is another object of the present invention to provide a factor VIII with an increased efficacy in factor VIII clotting assays.
It is still another object of the present invention to provide a factor VIII that is stable at a selected pH and physiologic concentration.